This invention relates to a corona discharge device for use in xerographic reproduction machines and, more particularly, to an improved control therefor which utilizes the voltage on either the coronode or the conductive shield of the corona discharge device for controlling the charge level on the photoconductive surface.
In xerographic reproduction copying machine, a predetermined uniform charge is normally placed on the surface of a photoconductor in preparation for imaging. The charged photoconductor or photoconductive surface is then exposed to a light image in order to form a latent electrostatic image. The latent image is then rendered visible by applying an electrostatically attractable marking medium conventionally referred to as toner with subsequent transfer of the toner image to a copy sheet. Following transfer, the image bearing sheet is subjected to a combination of heat and pressure or solely pressure to permanently fix or fuse the image to the copy sheet.
The charge level on the photoconductor of the machine is critical to the production of good quality copies. Thus, it is desirable to check, either intermittently or continuously, the charge level on the photoconductive surface for the purpose of making adjustments to the power supply for the corona discharge device. One device which may be used to measure the charge on the photoconductive surface comprises an electrometer. Since it is generally considered desirable to avoid placing an element in physical contact with the moving photoconductor except where absolutely necessary for fear of damaging or scratching the fragile surface of the photoconductor, electrometers employ a rather sophisticated and expensive capacitance type probe which permits probe placement adjacent to but out of physical contact with the photoconductive surface. Electrometer probes can also be positioned under the corona generator to measure the output thereof. Examples of such systems include those disclosed in U.S. Pat. No. 3,835,380 issued Sept. 10, 1974 to T. J. Webb and the reference listed therein; and U.S. Pat. Nos. 3,586,908 to R. Vosteen, 3,678,350 to S. Matsumoto et al. and 3,667,036 to N. Seachman. The charge measurement made in this way can be beneficially utilized to control various corona generator outputs or the like. However, these electrometers obviously require the use of electrometers and they occupy valuable space around the imaging surface and can only measure the charge in the position in which they are located. It is not economically or spacially desirable to provide several electrometers for measuring the charge on the imaging surface downstream of most of the corona generators in a copying apparatus. Moving an electrometer between different locations takes time and does not allow simultaneous measurements.
The space and expense problem of multiple electrometer probes is addressed in U.S. Pat. No. 3,950,680 issued in the name of Michaels et al. As stated therein, in that system the portion of each corona generator current going to its conductive shield is subtracted from the total input current supplied to that corona generator to provide a measurement of the current actually going from the corona generator to the imaging surface or plate. This is based on the principle that the total input current supplied to the corona generators must go to either the imaging surface or the shield and that if the shield current is electrically floated slightly above ground it can be fed back and subtracted to achieve the measurement of the true plate (imaging surface) current and therefore, the current applied charged. A current measuring device is utilized in a circuit comprising only the photoreceptor.
Another method of controlling the charge on the photoconductive surface is to develop a test image on the photoconductive surface in the inter-document area and to utilize an infrared densitometer in conjunction with the test image to develop an electrical output which is useful in controlling the charge level of the photoreceptor. Such a method is disclosed in U.S. Pat. No. 4,318,610 issued in the name of Robert E. Grace. As will be appreciated by those skilled in the art a test patch creates or represents a stress condition for the cleaning system of the reproduction apparatus and therefore may not be desirable for some applications.
Still another method of measuring or controlling the charge on a photoconductive surface utilizes a roller probe which is physically in contact with the photoconductive surface. As suggested in U.S. Pat. No. 3,887,845 issued to Robert J. Michatek, the roller probe can be the bias transfer roll in a machine where a bias transfer roll is utilized. Otherwise, the roller probe can be a separate roller. The obvious disadvantage of utilizing a bias transfer roll is in machines that do not utilize a bias transfer roll. Also, the spacial problems associated with electrometers are also inherent in roller probes.